1. Field of the Invention
The present invention relates to an image sensor for use in an image scanner or facsimile machine to read an original image.
2. Description of the Prior Art
An image sensor is composed of a plurality of image sensor chips; it reads an original image by means of a plurality of pixels provided on the image sensor chips, and outputs image data pixel by pixel successively.
In an image sensor chip, a switching element is provided on the output side of each of a plurality of photodiodes (pixels) arranged in a line. As one switching element after another is turned on by the use of switching pulses, the electric charge produced in each photodiode is successively transferred to and temporarily stored in a single capacitor until the stored charge is eventually output as a voltage. Meanwhile, it is essential that the charge from one photodiode be cleared, i.e. discharged, from the capacitor without fail before the switching element for the next photodiode is turned on.
This clearing of the capacitor is usually achieved by turning on a switching transistor provided in parallel with the capacitor. However, in an image sensor chip in which one end of the photodiode is connected to a power source line, when the switching element is turned on with a switching pulse, there occurs a ringing, i.e. oscillatory variation that settles as time passes, in the voltage on the power source line. This results from the voltage regulating action of a constant voltage source circuit as is widely used to produce from an unregulated direct-current voltage a constant voltage to be supplied to the power source line. More specifically, such a constant voltage source circuit reacts to variation in its output voltage, such as occurs when the switching element is abruptly turned on, by attempting to restore the normal output voltage. In particular, if the switching element is turned on substantially at the same time that the switching transistor is turned off, it is inevitable that the switching element is turned on just when the capacitor goes into a state in which it is ready to store electric charge. This causes an increase in "dark output voltage" (i.e. the output voltage that appears when the photodiode is not receiving light) under the direct influence of a ringing.
An increase in dark output voltage is undesirable in itself. In addition, the dark output voltage tends to be influenced by noise (variation) in the power source voltage; that is, noise (variation) in the power source voltage tends to cause noise (variation) in the dark output voltage. In an image sensor, which includes a plurality of image sensor chips, such noise (variation) in the power source voltage usually appears in different manners in different image sensor chips because of different wiring paths or other, and, quite undesirably, this causes the dark output voltage to vary from chip to chip.